US investigators are pressing for the Federal Aviation Administration to mandate modifications to Airbus A320-family power-distribution systems after revealing dozens of instances in which electrical bus failure resulted in the loss of cockpit displays and other functions.
The action follows an inquiry into a serious loss of instrumentation to a United Airlines A320 (N462UA) shortly after departure from New York Newark on 25 January. Three of the six electronic displays – the captain’s primary flight display, navigation display and upper centralised aircraft monitoring display – went blank.
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The landing gear did not retract, the radios were inoperative and the overhead panel was also blank. The aircraft returned to Newark and landed with several other systems not functioning – among them the transponder, collision-avoidance system and standby attitude indicator.
Preliminary investigations have traced the original failure to a fault in the AC1 bus, one of the aircraft’s two primary electrical distribution buses. Airbus last year published a service bulletin which enables the aircraft to reconfigure the power supply automatically, should a primary bus fail, but the FAA has not mandated its implementation.
The US National Transportation Safety Board says it is “especially concerned” that a bus failure during take-off or approach, particularly in instrument meteorological conditions, would present a greatly-increased workload and potential distraction to the crew at a critical phase of flight.
“Multiple system losses associated with an AC1 electrical bus failure can create a challenging situation for crews attempting to identify the nature of the fault and determine the best course of action for correcting the problem,” it says, recommending that Airbus’s power-reconfiguration change should be mandatory.
The NTSB says that the service bulletin is a “good and necessary step” towards mitigating the effects of bus failure, but adds that a prolonged resolution of the problem by the flightcrew could result in the unpowered standby attitude indicator ceasing to function after 5min.
Airbus has developed a further modification, featuring separate back-up power for the standby attitude indicator, to address this risk. While the NTSB says it is only available on new-build A320s, Airbus claims it is already available as a retrofit for in-service aircraft.
Data obtained by the NTSB from Airbus shows that, as of May last year, 49 events similar to that involving the United A320 had occurred in which bus failure resulted in the loss of flight displays and various systems.
Three-quarters of these involved the AC1 bus, and 17 of those resulted in the loss of five or six displays. The NTSB adds that the UK Air Accidents Investigation Branch has examined 14 additional display-blanking incidents involving AC1 bus failure.
In two of the events studied by the AAIB, the crew’s selection of the alternate bus feed did not result in the AC2 bus providing power. The NTSB says: “These incidents indicate that, even if a flight crew is able to effectively troubleshoot the malfunction, the problem may not be resolved quickly.”
It adds that the FAA should require all A320-family aircraft operators to develop new procedures, if necessary, and provide flightcrews with guidance and training to recognise and deal with the loss of flight displays in such circumstances.
Source: Flight International
